BEAT cream cheese, sugar and vanilla in large bowl with mixer until well blended. Add sour cream; mix well. Add eggs, 1 at a time, mixing on low speed after each just until blended. Pour over cake layer in pan.

BAKE 40 to 45 min. or until center is almost set. Run knife around rim of pan to loosen cake; cool before removing rim of pan.

MICROWAVE COOL WHIP and chocolate in microwaveable bowl on HIGH 1 to 1-1/2 min. or until chocolate is completely melted and mixture is blended, stirring after each minute. Cool 15 min.; slowly pour over cheesecake. Refrigerate 4 hours.